As road conditions have been improved, a tire for heavy load used, for example, in a truck or a bus has more opportunities .to run on a freeway and is used for a longer period. The durability of tire has been conventionally determined by a breakage such as a crack formed in a tire. However, nowadays, the value of a tire depends on the wear resistance because uneven wear tends to increase when a car runs on a freeway.
As such uneven wear, so-called center wear which occurs along the tire equator and so-called river wear which occurs along edges of circumferential grooves near tread edges have been particularly observed.
Conventionally, such uneven wear has been considered to be caused by a profile of the tread pattern.
However, the inventor determined out that, even in tires having same tread pattern, some produce a center wear and others a river wear and the wear amount is also different.
After an intensive study to find the causation factors, the inventor found that the center wear and river wear are not related only with tire's radius of curvature as conventionally believed, but also from other factors.
The inventor took note of a relationship between such uneven wear and the inflation amount of the tire's internal pressure, which has never been analyzed before. That is, the relationship between the center wear and the river wear and a dimensional difference between a state under 5% of a standard internal pressure, which is the minimum pressure required to maintain the shape of tire, and a state under standard internal pressure.
Thus, in an old-tire which exhibited considerable center wear and river wear and a new tire having the same shape and size, the inflation amounts were measured at the central area and shoulder area of the tread surface. As a result, it was confirmed that a tire with a considerable amount of center wear showed a larger inflation amount in the shoulder area than in the central area, while a tire with a considerable amount of river wear showed a smaller inflation amount in the shoulder area than in the central area.
Accordingly, the invention was completed by finding that the amounts of center wear and river wear could be reduced by controlling the difference in inflation amounts between the shoulder area and the central area, which further leads to a formation of a tire that produces neither center wear nor river wear.